The present invention relates to actuators or motors for producing motion.
By far the most commonly used actuators for motion control are inductive in nature. Examples are linear and rotary solenoids, brushed and brushless dc motors, brushless linear dc motors, and stepper motors. These inductive actuators are appropriate in velocity control applications with relatively low torque requirements. However, even there, the stiffness and bandwidth of an inductive motor are limited by properties of the magnetic coupling between the permanent magnet and the windings.
In many applications, a high force, quasi-static position actuator is desired. Achieving high torque outputs from an electric motor presents a variety of mechanical and electrical problems. Often, to obtain high torque outputs, some form of mechanical transmission is employed. However, the transmission reduces the actuator bandwidth and contributes to mechanical losses and backlash. In addition to transmission concerns, the motor itself has limitations, in that significant currents have to pass through the motor windings to increase motor torque outputs when the motor is operated close to stall. This causes high power dissipation through the winding resistance and results in a corresponding need to transfer the generated heat away. Further, the design and operation of inductive motors is complicated by the need to commutate the magnetic field. Commutation introduces significant torque ripple at low velocity and degrades overall torque output. Electrical commutation, as used in brushless motors, requires a motor position sensor whose output is fed back to a relatively complex controller. In a brushed motor, high currents at low velocities cause arcing of the commutation brushes and greatly reduce motor life. Thus, electric motors have inherent limitations.
From a purely theoretical point of view, capacitive devices such as piezoceramic actuators exhibit much more desirable mechanical and electrical characteristics. They have a very efficient coupling of energy from applied charge to mechanical strain, which results in a high bandwidth, a large force output and negligible resistive heating. The actuator stiffness is determined by the modulus of the ceramic material used for the actuator, rather than by an inherently weak magnetic coupling. Because these elements are capacitive in nature, they draw their least current at low or zero rate of displacement. Furthermore, a direct correspondence exists between actuator voltage and resultant position, without the need for commutation. Piezoceramic actuators, however, have historically been limited to extremely low displacement precision applications such as mirror control, ink jet nozzles, ultrasonic medical devices, high frequency audio speakers and miniature valves, where motions of only a few thousandths of an inch are needed.
Piezoceramics are commercially available in a variety of configurations, such as plates, tubes and stacks. Composite actuators, such as bimorphs, can be made by sandwiching a metal shim between two thin piezoceramics which are oppositely poled. When a voltage is applied to the bimorph, one piezoceramic expands while the other contracts, introducing a bending motion and/or bending moment of greatly amplified displacement into the composite element.
Several prior patents have been issued for hybrid devices, wherein electrically actuated elements that change dimension in response to an applied electrical drive signal are used to displace fluid for driving a hydraulic ram. Among such patents are U.S. Pat. No. 3,501,099 of Benson and U.S. Pat. No. 5,055,733 of Eylman. Other patents, such as U.S. Pat. No. 4,995,587 of Alexius show mechanical arrangements for amplifying the displacement so produced. However, to the applicant's knowledge, this prior art has not specifically addressed the particular mechanical properties of piezoelectric elements, other than, for example, their general benefit of electrical actuation and their usual limitation of small actuation displacement. In addition, this art has not achieved constructions which optimize the efficiency of a hybrid actuator, or which substantially outperform a conventional actuator.
Accordingly it would be desirable to provide a hybrid electrohydraulic actuator construction of enhanced electromechanical efficiency and performance characteristics.